There is a continuing industry demand for increasing power density, which means more power transferred in a given volume. A method for increasing the power transfer through the converter is to increase the switching frequency in order to minimize the size of magnetics and capacitors. Using prior art topologies such as forward or flyback, which employ “hard” switching techniques, makes high frequency operation less efficient. The switching losses associated with switch elements, which turn on when there is a voltage across them, are proportional with the switching frequency. An increase in switching frequency leads to an increase in switching losses and an increase in level of electromagnetic interference (EMI).
In order to overcome limitations in switching speeds, the prior art has devised a new family of resonant and quasi-resonant converters. In the case of quasi-resonant converters, the prior art technique consists of shaping the current or voltage to become half-sinusoidal and to perform the switching when the current or voltage reaches zero. The reactive elements, which contribute to shaping the current or voltage, are part of the basic circuit and are considered undesirable in classic topologies. An example of one such circuit can be found in Vinciarelli, “Forward Converter Switching at Zero Current”, U.S. Pat. No. 4,415,959. The technique utilized by Vinciarelli consists of adding a resonant capacitor across the fly wheeling diode to create a resonant circuit in combination with the leakage inductance of the transformer. During the ON time of the main switch, a current charges the resonant capacitor. When the current reaches zero, the main switch turns OFF in the primary of the transformer. The output inductor discharges the resonant capacitor, transferring the energy to the load. This topology exhibits several drawbacks which limit its utilization to power under 200 W.Another family of quasi-resonant converters which switch at zero voltage is described by F. C. Lee in High Frequency Power Conversion International Proceedings (April 1987), Intertec Communications, Ventura, Calif. These prior art circuits operate similarly to those described above with the exception that the main switch turns ON and OFF at zero voltage. Despite the advantage this has at the main switch the frequency modulation which is required for controlling the output power makes this topology unattractive.
New topologies which are refereed to as “Soft Transition Technologies” were developed, in order to eliminate the limitations associated with quasi-resonant and resonant converters, but to still maintain the advantage of soft commutations for the switching elements. Such technologies are described by I. Jitaru in “Fixed Frequency Single Ended Forward Converter Switching at Zero Voltage,” U.S. Pat. No. 5,126,931, and in “Square Wave Converter Having an Improved Zero Voltage Switching Operations,” : U.S. Pat. No. 5,231,563. Using these topologies the converter operates at constant frequency, modulating the power by varying the duly cycle, the current and voltages on the switching elements are square-wave to decrease the current and voltages stress, the transitions are done at zero voltage conditions, and the power is transferred to the output, both during the ON time and OFF time.
These latest topologies have proven superior in respect of efficiency over the previous resonant and Quasi-resonant topologies. However, the parasitic elements of the circuit such as leakage inductance and stray inductance, will negatively affect the efficiency due the circulating energy contained in these parasitic elements. Due to the inter-winding capacitance of the transformer the common mode noise will be injected into the secondary. In planar, low profile magnetics required for low profile packaging the inter-winding capacitance is larger, and as a result the common mode noise injected via parasitic capacitance is larger.